For the purposes of controlling a colored background (rectangle, square, patch, etc), it is merely necessary to control three variables, e.g. the red (R), the green (G) and the blue (B). However, the three variables of this example correspond to notions which are only difficultly perceptible by the human eye. Thus, in certain known means, a brightness control is brought about by a potentiometer, while there is a double control by means of a "joy-stick", which generates a voltage "square" by controlling two potentiometers. This "square" is likened to the color triangle or chromaticity diagram and one point of the triangle is associated with each position of the joy-stick. However, in this case, the sensitivity of the hue or tint control varies as a function of the zones, because of the nature of the triangle-square transformation.
Moreover, in the known constructions, the possibilities of modulating the hue and saturation are limited, because in particular it is not possible to act on the hue without modifying the saturation and vice versa.